a) Field of the Invention
The present invention relates to a lens system to be used in optical instruments and an image pickup apparatus which is equipped with this lens system.
b) Description of the Prior Art
In recent years where optical instruments such as video cameras and digital cameras prevail, zoom lens systems which can vary focal lengths thereof are adopted at a high ratio as lens systems for these optical instruments.
However, a zoom lens system ordinarily requires six to seven lens elements even when it is to be composed of a small number of lens elements or ten or more lens elements when it is to be composed of a large number of lens elements, thereby constituting a hindrance to compact configuration of video cameras and digital cameras. Further, it is common to use four lens elements for composing a zoom lens system for silver salt cameras in a small number of lens elements or ten or more lens elements for composing such a lens system of a larger number of lens elements, thereby also constituting hindrances to compact configuration of lens systems and reduction of manufacturing costs thereof.
On the other hand, lens systems which are disclosed by Japanese Patents Kokai Publication No. Sho 63-276013 and Kokai Publication No. Hei 2-56515 are known as conventional examples of zoom lens systems which are composed of small numbers of lens elements.
The zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 63-276013 is composed of four lens elements using a radial type gradient index lens element. Further, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-56515 is composed of two lens elements using a radial type gradient index lens element.
The zoom lens system disclosed by Japanese Patent Kokai Publication No. Sho 63-276013 is composed of two lens units: in order from the object side, a positive lens unit and a negative lens unit. This zoom lens system has an exit pupil which is located close to the lens system due to the composition described above, thereby being undesirable for use in an electronic image pickup optical system of a video camera or a digital camera since the lens system does not allow a principal ray to be incident perpendicularly onto an image pickup surface. In other words, this zoom lens system is largely deviated from a telecentric condition.
Further, the zoom lens system disclosed by Japanese Patent Kokai Publication No. Hei 2-56515 is composed of a number of lens elements as few as two, but is expensive since both of the lens elements are radial type gradient index lens elements and further makes it difficult to manufacture a material for a radial type gradient index lens element which is disposed on the object side since it has a negative refractive power.